Aqueous polyurethane dispersions are widely used as binders for aqueous paints. Among these paints, one-pack systems have been known which dry physically (by evaporation of the water) or by oxydative crosslinking (air-drying polyurethanes such as those described in EP 1 026 186 A1), the formation of a film being promoted where appropriate by addition of thermally activated crosslinking agents (one-pack systems or stoving paints which may be mixed well before use, and are cured with melamine resins or blocked isocyanates at temperatures above 120° C.); and two-pack systems, where hydroxy-functional binder constituents are mixed with non-blocked polyfunctional isocyanates immediately prior to application and the crosslinking reaction sets on at room temperature (or at an elevated temperature in order to accelerate the cure). In the latter case it is also possible to emulsify the non-blocked isocyanate in the aqueous dispersion of the hydroxy-functional binder component, without any premature reaction of the two components.
The basis of common polyurethane dispersions are mostly polyols selected from the group consisting of polyether and polyester polyols. These are reacted with poly-functional isocyanates, and modified with hydrophilic components such as polyethylene glycol, or anionogenic compounds such as hydroxyacids wherein the acid group does not largely react with isocyanates.
When such aqueous polyurethane dispersions are used to provide coatings on wood, the coatings thus produced are usually inferior to those made from solvent borne systems with regard to the wood wetting properties which leads to enhancement of the immanent structure found in natural substrates such as wood.